resistance form- restorative dentistry (dr. joaquin masoud c. shafiee)

7/29/2019 resistance form- restorative dentistry (dr. joaquin masoud c. shafiee)

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Obtaining the Resistance Form |

Resistance Form

Primary Resistance Form Definition:PrimaryResistance form is that shape and placement ofthe cavity walls to best enable both the tooth andrestoration to withstand, without fracture the

stresses of Masticatory forces deliveredprincipally along the long axis of the tooth.

Resistance form is the design of a cavity in such away that the remaining tooth substance and the

restorative material can withstand masticatorystress.


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To achieve this ,the prepared cavity

should possess the following 6

attributes discussed below: Flat Floor

Adequate bulk of the restorative material

Absence of weak cusps or marginal ridges

Occlusal cavity margins in areas not subjected toexcessive occlusal trauma . In practice one-quarter (1/4) of the intercuspal width

Flat floor at right angles to the line of stress Walls of the cavity parallel to the direction of the

stress


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1) Flat floor: A flat pulpal floor should be

given while the cavity is being prepared to

avoid unwanted stresses and forces on the

pulpal floor.


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2) Adequate bulk of the restorative

material:

Bulk of the material should be 1.5-2mm theprepared cavity should be deep enough to

take adequate bulk of the restorative material

capable of withstanding masticatory stress.

The bulk required will depend on the flexural

strength of restorative material


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. In the case of amalgam it is estimated that a

minimum of 1.5-2mm thickness of the

restorative material is required to withstand

masticatory stress.


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3) Absence of weak cusps or marginal

ridges:

After cavity preparation, the tooth should notbe left with any weak cusps or marginal ridges.Any weak cusp must be removed and restored

with a metallic restorative material, such as silveramalgam or dental gold. If a marginal ridge isfound to be too weak in the cause of an occlusalcavity preparation, a Class II cavity may have to

be prepared instead, so as to eliminate the weakmarginal ridge.


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This is particularly indicated where the ridge is

only of enamel thickness and unsupported by

sound dentine


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4) Occlusal cavity margins in areas not

subjected to excessive occlusal

trauma . In practice one-quarter (1/4)of the intercuspal width

The cavity should be designed that the occlusalmargins of the cavity are in areas not subjected toexcessive occlusal trauma, otherwise the enamel

wall of the cavity and/or the margins of therestorative material may fracture


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. In practice, this may be achieved by placing

an occlusal margins of a cavity about one-

quarter (1/4) of the intercuspal distance.

Note, that efforts should always be made to

conserve sound tooth tissue.


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5) Flat floor at right angles to the line of

stressThe floor of prepared cavity should beflat and right angles to the line of occlusal

stress, which is usually in the direction of long

axis of the tooth. Sound tooth tissue should,

however, not be removed simply to obtain a

flat pulpal floor of prepared cavity


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6) Walls of the cavity parallel to the direction

of the stressTo achieve this , the walls of thecavity are prepared parallel to the

corresponding tooth surfaces.


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Fundamental principles involved to

Obtain Primary Resistance Form are:

Box shape or mortise shaped with flat floor, whichhelps the tooth to resist occlusal loading by virtue ofbeing at right angles to the forces of mastication.

Slightly curved than acute line angles decrease thestress concentration of stresses and hence reduce theincidence of fracture.

Conservation of strong cusps and ridges with sufficientdentin support.Weakened areas should be included incavity preparation to prevent fractures (capping of theweakened cusps).


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To provide enough thickness of restorative

material to prevent fracture under load.

Slight roundening of the line angles to prevent

stress concentration.


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resistance form- restorative dentistry (dr. joaquin masoud c. shafiee)

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